1. Technical Field of the Invention
The invention relates to the decoding of a radio frequency transmission channel conveying coded digital information. It applies advantageously but not limitingly to receiving and decoding terrestrial digital television signals.
The invention thus relates more especially to tuners, in particular those of the double-conversion type, such as for example those incorporated into the modulation/demodulation devices (modems) intended for receiving terrestrial digital signals, that is to say those conveyed by cable.
In the case of cable reception, the input frequencies of the signal are spread over a span of from 47 MHz to 864 MHz in intervals of 6 or 8 MHz. This represents 135 channels. An extension to 1 GHz is moreover envisaged in the future.
2. Description of the Related Art
At present, modems are equipped with a head filter. For reasons of economy, the elimination of this head filter is envisaged. This consequently implies that 135 channels can enter the tuner, this representing a considerable input dynamic range. It is consequently necessary to control the gain of the input amplifier stage of the modem so as to avoid nonlinearities which would be caused by a saturation phenomenon, and to do so without excessively degrading the signal-to-noise ratio.
At present, the power of the signal is measured after analog/digital conversion and the gain of the tuner is controlled on the basis of this power measurement according to a predetermined control law. However, the adjustment of the gain does not in any way depend on the power of the signal entering the tuner, but only on the power of the desired signal.